In a holographic recording medium, holograms (known also as gratings) are recorded from the interference of signal and reference beams within a photosensitive medium. A medium for holographic applications should be capable of recording the spatial intensity variation as refractive index modulation throughout the volume. A number of factors determine the hologram's strength and performance including a combination of: the refractive index modulation or fringe contrast; the thickness of the medium; and the optical losses resulting from scattering and absorption within the medium. In order to address the needs of holographic systems (for example, holographic filters, holographic data storage, holographic lenses) the thickness of the photosensitive medium must generally be 200 μm or greater. Such media are generally termed ‘thick media’, and should exhibit low scattering, high recording sensitivity, stable image fidelity, low volume shrinkage and high dynamic range.
The photopolymers in one class that has been studied comprise a homogenous mixture of a polymeric binder and at least one photopolymerizable monomer and/or oligomer. The holograms are created by component segregation (through diffusion of monomers) and local photo-polymerization of one or more components. These interference fringes create regions of low and high refractive index, and it is desirable to maximize this difference to maximize fringe contrast. However, practical limits to the available refractive index contrast will be established by other related parameters that define the overall optical performance of the media. For example, one of these considerations, reliability, is impacted strongly by the degree of component segregation in making up the fringe. Consequently, the choice of components such as the binder for use in this medium requires more consideration than just the refractive index difference.
Colvin et al., in U.S. Pat. No. 6,165,648, developed a medium containing photopolymerizable acrylate oligomers without an inert component. This material is based on free-radical polymerization and it contains low molecular weight monomers, for example isobornyl acrylate, and has low sensitivity because of the high cross-link density of the polymer network. The dense state of the polymer impedes diffusion of recording monomers lowering photosensitivity and dynamic range.
The prior art discloses the use of solvents either as a neutral binder [W. J. Tomlinson et al., Multicomponent photopolymer system for volume phase holograms and grating devices, Appl. Opt., vol. 15(2), pp. 534-541, 1976] or as an inert component [T. Smimova et al., “The effect of structural-kinetic features of hologram formation on holographic properties of photopolymers”, Semiconductor Physics, Quantum Electronics Optoelectronics, 2004, v. 7, n. 3, pp. 326-331] in combination with photopolymerizable oligomers or monomers to form a holographic medium. In this context the inert component is not involved in the polymerization reaction, but may perform some other function such as maintaining compatibility, modifying reaction rates, or engineering a specific refractive index in the medium. However, photopolymer systems containing solvents can show temperature instability and have high light scattering making them unacceptable for preparation of thick holographic medium. The solvents disclosed by Smimova have a low boiling point and low molecular weight that leads to temperature instabilities and the formation of droplets with sizes larger than the recording wavelength. These solvents also have high coefficients of thermal expansion (CTE ˜10−3/° C.) that can significantly reduce reliability of the media.
Cetin et al., in U.S. Pat. Nos. 6,784,300 and 7,070,886, developed a holographic medium based on a non-volatile and viscous inert oligomeric binder mixed with cationic photopolymerizable monomers/oligomers. These cationic ring opening polymerization (CROP) systems have relatively high sensitivity, high dynamic range and low shrinkage because volume change is compensated by opening of the epoxy ring during holographic recording. However, it is known that photo-initiated cationic polymerization has a relatively low polymerization rate compared to very reactive radical photopolymerizable acrylic systems (Berlin) and that there are disadvantages in that the holographic optical medium stability before exposure is poor and the recording speed depends greatly on temperature [C. Decker, K. Moussa, Journal of Polymer Science: Part A: Polymer Chemistry, vol. 28, pp. 3429-3443 (1990)].
Therefore, a need still exists for holographic recording media that exhibit high performance and reliability when formed in layers of about 200 μm and greater.